Senior Bush administration officials intensified the effort to make the case for military action against Saddam Hussein today, with testimony by Secretary of State Colin L. Powell and the director of central intelligence, George J. Tenet, linking Iraq and Al Qaeda.

Mr. Powell seized on a new audiotape believed to be of Osama bin Laden, urging Muslims to help Baghdad defend itself against an American attack, as evidence that the Qaeda leader was ''in partnership with Iraq.''

In testimony before the Senate Intelligence Committee today, the F.B.I. director, Robert S. Mueller III, focused on the threat posed by Al Qaeda in the United States. He said several hundred Islamic militants linked to Al Qaeda were in the country, with some organized in cells that could be ordered to carry out terrorist attacks here.

Mr. Powell told the Senate Budget Committee that the bin Laden statement, broadcast this afternoon by Al Jazeera, an Arabic language television network, demonstrated that the United States could not contain Iraq through more aggressive weapons inspections or an enlarged United Nations presence.

The administration's attempt to tie Iraq to Al Qaeda also included the most explicit public statement yet by Mr. Tenet, who told the Senate intelligence committee that intelligence officials had unearthed powerful evidence showing a connection.

Mr. Tenet's testimony was especially noteworthy because some Pentagon and White House officials had privately complained that the C.I.A. was too reluctant to conclude there was an Iraq-Qaeda link.

Today Mr. Tenet said Iraq is ''harboring'' senior members of the Qaeda network like Abu Musab al-Zarqawi, who, Mr. Tenet said, assembled a terror cell last year from a base of operations in Baghdad.

In the tape, which administration officials said they first learned of on Monday evening, an unidentified voice urged Muslims to defend Iraq against an American-led attack. Today, intelligence officials said they had not yet conducted advanced voice authentication tests, but believed that the voice was Mr. bin Laden's.

The voice called for attacks on the United States and Israel and said any cooperation with Washington by a Muslim would be anti-Islamic, which appeared to be a warning to the governments of Morocco, Saudi Arabia, Nigeria and Yemen, which have aided the United States.

Mr. Powell, who broke the news about the tape several hours before the broadcast, said, ''You will be seeing as this day unfolds where once again he speaks to the people of Iraq and talks about their struggle and how he is in partnership with Iraq.''

He added, ''This nexus between terrorists and states that are developing weapons of mass destruction can no longer be looked away from and ignored.''

Later, Mr. Powell's spokesman, Richard A. Boucher, said the tape had confirmed ''that bin Laden and Saddam Hussein seem to find common ground.'' For Mr. Tenet, who sat silently behind Mr. Powell on Feb. 5 when the secretary presented the Bush administration's case against Iraq to the United Nations Security Council, the hearing today presented the country's intelligence chief with his first opportunity to publicly assess the intelligence on Saddam Hussein's leadership.

Mr. Tenet said the C.I.A. had carefully worded its accusations about Iraq, but he strongly supported the Bush administration's conclusion that an Iraq-Qaeda link had been firmly established even as some European officials, and some intelligence analysts within his own agency, have expressed skepticism about whether the information showed a direct Iraqi tie to Al Qaeda.

''Iraq has in the past provided training in document forgery and bombmaking to Al Qaeda,'' Mr. Tenet said. ''It has also provided training in poisons and gases to two Al Qaeda associates. One of these associates characterized the relationship he forged with Iraqi officials as successful.''

Mr. Tenet seemed to be trying to rebut skeptics among the allies and within his own agency. ''This information is based on a solid foundation of intelligence,'' he said. ''It comes to us from credible and reliable sources. Much of it is corroborated by multiple sources, and it is consistent with the pattern of denial and deception exhibited by Saddam Hussein over the past 12 years.''

At the White House, the president's spokesman, Ari Fleischer, said the tape ''underscores what the president and Secretary Powell have said about Al Qaeda linking up with Iraq.''

He continued: ''Secretary Powell already demonstrated proof of the link at the operative level. This adds further proof that the leader of the operatives, if this is indeed Osama bin Laden, is exhorting them to link up with 'our mujahedeen brothers in Iraq.' ''

In the past, White House officials have discouraged news organizations from broadcasting Mr. bin Laden's messages because of the danger that they might contain coded messages to his followers.

The Fox News Channel took Mr. Powell's statement about the tape as a signal to play it live as it was broadcast on Al Jazeera.

''Powell basically parsed the thing for the world, which I think took the curse off of it,'' said John Moody, the Fox News Channel's senior vice president.

The 16-minute tape in Arabic urged Iraqis to wear down the American forces through extended urban warfare.

''Our mujahedeen brothers in Iraq, do not be horrified about what America is propagating about their force, smart bombs, laser-directed bombs,'' the speaker on the tape said.

''These smart bombs have no effect in mountains and caves, forests,'' he continued. ''They need very obvious targets to be effective. Camouflaged targets cannot be hit by either smart bombs or stupid bombs.''

Mr. Tenet said Mr. bin Laden's followers appeared to focus their terror planning on specific regions. ''The information we have points to plots aimed at targets on two fronts -- in the United States and on the Arabian peninsula,'' he said. ''It points to plots timed to occur as early as the end of the hajj, which occurs late this week.''

''And it points to plots that could include the use of a radiological dispersion device as well as poisons and chemicals,'' Mr. Tenet added, referring to devices like a ''dirty bomb'' that detonates with a conventional explosive, throwing radioactive material into the air.

The hearing, an annual assessment of global threats, centered on Al Qaeda, Iraq and North Korea.

In his testimony, Mr. Mueller told the Senate intelligence committee that the effort to identify Qaeda cell members is the country's ''most serious law enforcement challenge.''

Speaking of Qaeda followers in the United States, Mr. Mueller said: ''The focus of their activities centers primarily on fund-raising, recruitment and training. Their support structure, however, is sufficiently well-developed that one or more groups could be ramped up by Al Qaeda to carry out operations in the U.S. homeland.''

In his testimony, Mr. Tenet seemed to agree, defending the decision last Friday by the Bush administration to ratchet up the official threat level to Code Orange, saying it was based ''not on idle chatter on the part of terrorists and their associates.''

''It is the most specific we have seen, and it is consistent with both our knowledge of Qaeda doctrine and our knowledge of plots this network -- and particularly its senior leadership -- has been working on for years,'' Mr. Tenet said. ''The intelligence community is working directly, and in real time, with friendly services overseas and with our law enforcement colleagues here at home to disrupt and capture specific individuals who may be part of this plot.''

Mr. Tenet offered no further specifics about the plot, but his testimony reflected the rapidly rising tensions around the world in a period when the United States continues preparations for possible military action in Iraq, North Korea has threatened to resume its nuclear program and Iran has disclosed that it is nearly ready to enrich uranium to fuel its nuclear energy program.

''We see disturbing signs that Al Qaeda has established a presence in both Iran and Iraq,'' Mr. Tenet said. ''In addition, we are concerned that Al Qaeda continues to find refuge in the hinterlands of Afghanistan and Pakistan.''